A III group nitride compound semiconductor has an energy direct transition-type bandgap corresponding to a visible to ultraviolet radiation range and can emit light with high efficiency, and is therefore manufactured as an LED and LD. The III group nitride compound semiconductor also has a potential to provide characteristics that are not obtained by a conventional III-V group compound semiconductor.
Generally, the III group nitride compound semiconductor is produced by an organic metal-organic chemical vapor deposition (MOCVD) method using trimethyl gallium (TMG), trimethyl aluminum (TMA) and ammonia (NH3) as raw materials. The MOCVD method is a method in which a carrier gas is mixed with vapor of a raw material and the mixture is transported onto the surface of a substrate, and then the raw material is decomposed by a reaction with the heated substrate, and thus crystal growth occurs.
A single crystal wafer of the II group nitride compound semiconductor is not commercially available and a method of growing a crystal of the III group nitride compound semiconductor on a single crystal wafer made of a different material is commonly used. There is a large lattice mismatch between a different type of a substrate and the III group nitride compound semiconductor crystal, which is epitaxially grown on the substrate. For example, 16% of the lattice mismatch exists between sapphire (Al2O3) and gallium nitride (GaN), while 6% of the lattice mismatch exists between SiC and gallium nitride. When such a large lattice mismatch exists, it is difficult to epitaxially grow the crystal directly on the substrate and a good crystal cannot be obtained even when epitaxial growth is performed. Therefore, when a III group nitride compound semiconductor crystal is epitaxially grown on a sapphire single crystal substrate or a SiC single crystal substrate by an organic metal-organic chemical vapor deposition (MOCVD) method, as disclosed in Japanese Patent No. 3,026,087 and Japanese Unexamined Patent Publication (Kokai) No. 4-297023, a method has been used of firstly stacking a layer called a low-temperature buffer layer composed of aluminum nitride (AlN) or aluminum gallium nitride (AlGaN) on a substrate and then epitaxially growing a III group nitride compound semiconductor crystal thereon at a high temperature.
Some reports have been made with respect to a technique of forming, as a buffer layer, a layer of AlN using a method other than a MOCVD method and forming another layer using a MOCVD method. For example, Japanese Examined Patent Publication (Kokoku) No. 5-86646 describes a technique of using a MOCVD method to grow a crystal with the same composition on a buffer layer formed by RF sputtering. However, Japanese Patent No. 3440873 and Japanese Patent No. 3,700,492 describes that a good crystal cannot be safely obtained only by the technique described in Japanese Examined Patent Publication (Kokoku) No. 5-86646. In order to stably obtain a good crystal, Japanese Patent No. 3,440,873 described that a buffer layer is annealed in a mixed gas of ammonia and hydrogen after growing the buffer layer, and Japanese Patent No. 3,700,492 describes that it is important that a buffer layer is formed by DC sputtering at a temperature of 400° C. or higher.
Research into producing a III group nitride compound semiconductor crystal by sputtering has been carried out. For example, Japanese Unexamined Patent Publication (Kokai) No. 60-39819 describes that a layer of GaN is directly formed on a substrate by sputtering for the purpose of laminating a GaN having high resistance. The conditions used are as follows: attainable pressure: 5×10−7 to 10−8 Torr, gas circulating in chamber: Ar and N2, gas pressure during sputtering: 3 to 5×10−2 Torr, RF voltage: 0.7 to 0.9 kV (power: 20 to 40 W), distance between substrate and target: 20 to 50 mm, and substrate temperature: 150 to 450° C. However, it is not an object of these applications to produce an underlying layer of a light emitting device and there is no description concerning the deposition of a layer on the film.
Collected Papers of 21st Century Combined Symposium, Vol. 2nd, p 295 (2003) describe that a GaN film was formed on Si (100) and Al2O3 (0001) by high-frequency magnetron sputtering using an N2 gas. The deposition conditions are as follows: total gas pressure: 2 m Torr, power: 100 W, and substrate temperature: ranging from room temperature to 900° C. According to the figures in the document, a target is opposed to a substrate in an apparatus used.
In Vacuum, Vol. 66, p 233 (2002), a layer of GaN is formed using an apparatus wherein a cathode is opposed to a target and a mesh is interposed between a substrate and a target. In this document, the deposition conditions are as follows: pressure in N2 gas: 0.67 Pa, substrate temperature: 84 to 600° C., power: 150 W, and distance between substrate and target: 80 mm.